Retreaded tires provide an economical way to gain additional use from tire casings after the original tread or retread has become worn. According to a conventional method of retreading, sometimes referred to as cold process retreading, worn tire tread on a used tire is removed to create a buffed, generally smooth treadless surface about the circumference of the tire casing to which a new layer of tread may be bonded.
The tire casing is then typically inspected for injuries, some of which may be skived and filled with a repair gum while others may be severe enough to warrant rejection of the casing. After completion of the skiving process, the buffed surface may be sprayed with a tire cement that provides a tacky surface for application of bonding material and new tread. Next, a layer of cushion gum may be applied to the back, i.e., the inside surface of a new layer of tread, or alternatively, the layer of cushion gum may be applied directly to the tacky surface on the tire casing. Conventionally, the cushion gum is a layer of uncured rubber material. The cushion gum and tread may be applied in combination about the circumference of the tire casing to create a retreaded tire assembly for curing. As an alternative, a length of tire tread may be wrapped around the tire casing with the cushion gum already applied. The cushion gum may form the bond between the tread and the tire casing during curing.
Following assembly of the tire casing, cement, cushion gum and tread, the overall retreaded tire assembly may be placed within a flexible rubber envelope. An airtight seal may be created between the envelope and the bead of the tire. The entire envelope tire assembly may be placed within a curing chamber and subjected to a vulcanization process that binds the materials together.
Tire buffers are generally used in tire retreading operations to remove the old, worn tread and to prepare the tire casing for receiving the new, replacement tread. More specifically, the tire buffers are used to remove excess rubber from the tire casing. The buffing of the casing can also be used to provide a substantially even texture on the crown of the casing and to shape the casing into a predetermined profile that is complementary to the new, replacement tread that is to be attached to the tire casing. Tire casings usually include a steel belt package (a package of steel belts or cables) underlying the road-engaging surface (e.g., the original tread) of the tire. During the buffing process, the casing must be buffed, generally to a predetermined characteristic crown radius corresponding to the upper contour of the belt package. In particular, the casing is buffed to leave only a predetermined thickness of material remaining over the top steel belt. In performing the buffing operation, it is important that the buffer not remove too much rubber from the casing and cut into the steel belt package. If any of the steel belts are damaged in any significant respect, the tire casing has to be scrapped.
Typically, tire buffers are automated and are programmed to remove rubber to the desired predetermined depth. To program the tire buffer, pilot skives are done with a hand tool in which the rubber is cut down to the steel belts in three equidistant narrow strips around the tire. This is done to find the approximate depth of the steel belts in that particular tire. This approximate depth is used as a guideline to program the automated tire buffer or as guide for a manual buffer. Unfortunately, this procedure is time-consuming and imprecise and if the tire buffer is set-up wrong it will result in damage to the steel belt package and scrapping of the tire.
The determination of the approximate depth of the steel belts is also occasionally done using an induction sensor that senses the distance between the sensor and the steel belts. However, the accuracy of the induction sensor is influenced by the amount of rubber on the tire and the construction of the steel belts and often can produce inaccurate results. One way to try to avoid such inaccuracies is by re-calibrating the induction sensor each time a tire casing is placed in the buffer. Yet, the recalibration of the induction sensor for each new buffing operation is time-consuming and does not foreclose the possibility of errors in measuring the depth of the steel belts.